Method and system for precisely positioning a waist of a material-processing laser beam to process microstructures within a laser-processing site

Abstract

A high-speed method and system for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site are provided. In the preferred embodiment, the microstructures are a plurality of conductive lines formed on a plurality of memory dice of a semiconductor wafer. The system includes a focusing lens subsystem for focusing a laser beam along an optical axis substantially orthogonal to a plane, an x-y stage for moving the wafer in the plane, and a first air bearing sled for moving the focusing lens subsystem along the optical axis. The system also includes a first controller for controlling the x-y stage based on reference data which represents 3-D locations of microstructures to be processed within the site, a second controller, and a first voice coil coupled to the second controller for positioning the first air bearing sled along the optical axis also based on the reference data. The reference data is generated by the system which includes a modulator for reducing power of the material-processing laser beam to obtain a probe laser beam to measure height of the semiconductor wafer at a plurality of locations about the site to obtain reference height data. A computer computes a reference surface based on the reference height data. A trajectory planner generates trajectories for the wafer and the waist of the laser beam based on the reference surface. The x-y stage and the first air bearing sled controllably move the wafer and the focusing lens subsystem, respectively, to precisely position the waist of the laser beam so that the waist substantially coincides with the 3-D locations of the microstructures within the site. The system also includes a spot size lens subsystem for controlling size of the waist of the laser beam, a second air bearing sled for moving the spot size lens subsystem along the optical axis, a third controller for controlling the second air bearing sled, and a second voice coil coupled to the third controller for positioning the second air bearing sled along the optical axis.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is related to provisional patent application entitled “Trajectory Generation And Link Optimization”, filed the same day as the present application. Also, this application is related to U.S. patent applications entitled “Precision Positioning Apparatus” filed on Sep. 18, 1998 and having U.S. Ser. No. 09 / 156,895, and “Energy Efficient Laser-Based Method and System for Processing Target Material” filed on Dec. 28, 1999 and having U.S. Ser. No. 09 / 473,926.TECHNICAL FIELD [0002] This invention generally relates to methods and systems for high speed laser processing (machining, cutting, ablating) microstructures. More specifically, this invention relates to methods and systems for precisely positioning a waist of a material-processing laser beam to process microstructures within a laser-processing site. Semiconductor memory repair is a specific application where precise positioning in depth of the beam waist of the laser beam...

AI Technical Summary

Benefits of technology

[0028] The step of providing may include the steps of reducing power of the material-processing laser beam to obtain a probe laser beam and utilizing the probe laser beam to perform the step of measuring.

[0029] Further in carrying out the above objects and other objects of the present invention, a system for precisely positioning a waist of a material-processing laser beam to dynamically compensate for local variations in height of microstructures located on a plurality of objects spaced apart within a laser-processing site is provided. The system includes a focusing lens subsystem for focusing a laser beam along an optical axis, a first actuator for moving the objects in a plane, and a second actuator for moving the focusing lens subsystem along the optical axis. The system further includes a first controller for controlling the first actuator based on reference data which represents 3-D locations of microstructures to be processed within the site, and a second controller for controlling of the second actuator also based on the reference data. The first and second actuators controllably move the objects and the focusing lens subsystem, respectively, to precisely position the waist of the laser beam and the objects so that the waist substantially coincides with the 3-D locations of the microstructures within the site.

[0044] The height information will preferably be obtained from the same laser and optical path used for processing, but with reduced power (with a modulator used to reduce the power and avoid damage to the surface).

[0070] A system of the present invention is able to operate with smaller spot sizes (which require better focus control) and thereby process devices with smaller geometry than prior memory repair systems due in part to superior focus control.

[0071] Dynamic Focus allows a system of the present invention to adapt to the non-parallel and non-planar topology that is typically found on real wafers and maintain acceptable focus over the full extent of a die site.

Problems solved by technology

This manufacture of smaller devices affects the geometry of the links allocated for laser redundancy.

A defocused spot reduces the energy that is delivered to the target link possibly leading to insufficient cutting of the link.

A defocused spot may also place more laser energy on adjacent links or on the intervening substrate leading to possible substrate damage.

At some level of defocus, the laser cutting process is no longer viable.

This presents a problem in that the wafers seldom are flat (planar) and parallel to the focal plane.

The process or sensor used to measure focus may exhibit errors.

The mechanism used to provide relative motion between the wafer surface and focal plane may exhibit errors.

The problem encountered at the time was due to the packaged die being significantly non-parallel to the surrounding package (typically pressure sensors).

One problem of this approach is that the auto-collimator worked best when it could be directed at a large “planar” object.

It would not be possible to find such a region on a typical IC found in memory repair applications.

In 1994, GSI developed a different approach to handle thin-film trimming on “tilted die.” The problem was again due to trimming on packaged IC (pressure sensors).

In this case, the specifics of the customer's device precluded the use of a tilting Z-stage.

Also, the absence of suitable target structures for the auto-collimator on certain customer's devices forced GSI to develop the multi-site focus algorithm.

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